In recent years the use of acoustic emission techniques has become a significant aspect of non-destructive testing. Acoustic emission as defined herein, and as understood by the trade, refers to a transient elastic stress wave generated within a material as the material is deformed under stress. The emission is considered transient in that it will rapidly subside if the applied load remains constant. However, if the material is placed under a dynamically increasing load, the elastic wave will not be transient, but will be continually generated as the load is increased.
The physical events occuring within a material which give rise to acoustic emissions differ according to the properties of the specific material. For instance, the physical events occuring within a ferrous material under strain differ from the events occuring in fiberglas under strain. The present invention is related to the use of acoustic emission to detect damage loading of fiberglas booms or similar fiberglas members. Consequently, only the acoustic emission associated with events produced in fiberglas need be discussed.
There are three significant events which produce acoustic emissions in fiberglas. The first is called matrix crazing. Matrix crazing may be defined as the cracking of the polyester layers within the fiberglas composition. A second event which produces acoustic emission in fiberglas is called debonding. Debonding may be defined as the glass fibers breaking away from the polyester layers. The third, and by far most significant event causing acoustic emission in fiberglas is fiber breakage. Fiber breakage may be defined as the physical snapping of a strand of glass fiber in the tension mode.
Each of the above events release acoustic emission energies having discrete amplitudes and frequencies. For this reason, it is possible in the acoustic emission monitoring of fiberglas to discriminate and identify those acoustic emissions which correspond to fiber breakage. This invention is related in part to an acoustic monitor which discriminates and identifies fiber breakage.
A phenomena known as the Kaiser effect is associated with acoustic emissions. The Kaiser effect occurs in all materials, and is characterized by the immediate irreversible characteristic associated with acoustic emissions whereby acoustic emissions will not occur until the previous maximum stress level experienced by the material is exceeded. This phenomena is valid for materials generally, provided stresses are kept below the yield point.
However, a unique phenomena occurs in fiberglas. At a stress level approximately one-half of the residual strength of the fiberglas member, the material will no longer exhibit the Kaiser effect. Any subsequent loading which exceeds the one-half residual strength point will produce acoustic emissions, even though the loading does not exceed the previous maximum load. Even more importantly, the acoustic emission is no longer transient when the load exceeds the point corresponding to approximately one-half of the residual strength. Thus, even though the load remains constant, acoustic emissions will continue if the load is above the one-half residual strength point. This is an important feature in the non-destructive testing of fiberglas members, particularly booms.
Fiberglas members may be used in many applications. It is particularly important to guard against the failure of fiberglas booms when the boom is being used to support personnel at high levels above the ground. This is a common situation found in aerial trucks or "cherry pickers" which employ fiberglas booms.
Acoustic emission techniques have previously been used for periodic testing of the fiberglas booms of aerial trucks. This periodic testing is performed by placing a piezoelectric transducer in contact with the boom to detect acoustic emissions. The boom is then subjected to a proof load normally a multiple of the rated load which will not intentionally be exceeded in the field. A common practice in the industry is to use a proof load which is a nominal 1.5 to 3 times the rated load. The proof load is applied to the boom and held constant to allow the transient acoustic emission, if any, to die out. If the material is then acoustically silent, the test shows that the non-Kaiser point is greater than the proof load. Since the non-Kaiser point occurs at just over one-half of the residual remaining strength, and the proof load may be approximately 1.5 to 3 times greater than the rated load, there exists a safety factor of approximately between 3 to 1 and 6 to 1 for the boom.
Such periodic testing of fiberglas booms is normally carried out on a yearly, semi-annual or quarterly basis using the above described techniques. However, this periodic testing program is somewhat cumbersome and requires that the equipment be taken out of field use during the time of testing. Furthermore, special load equipment and personnel are required to conduct the test. Perhaps the greatest shortcoming of periodic testing is that there is no protection during the interval between tests, creating the possibility that a crew could be using a dangerous piece of equipment for several months before it is revealed by the next periodic test.
My research into catastrophic failures of actual field equipment has revealed another significant feature of fiberglas booms. A new boom, if stressed to failure by increasing the load over a relatively short period of time will result in the boom failing in the compression mode. However, in the field failures which I observed, the booms all failed in the tension mode. There is a dangerous difference between the failure modes. In compression failure, the boom bulges at the failure site and undergoes a delayed bending. This would result in the occupants of a basket at the end of the boom being dropped to the ground at a rate slower than a free fall. However, failure in the tension mode results in a sharp snap of the boom and a free fall of the basket. Personal injury is almost certain.
My research on the failed booms indicates that the tension mode failures were caused by significant fiber breakage in the boom. The fiber breakage was caused by loading the boom above the non-Kaiser point for a significant period of time. This loading above the non-Kaiser point causes fibers to snap in the tension mode. The occurence of significant fiber breakage caused by such loading may be referred to as a "damage load". The amount of fiber breakage depends upon how much the load exceeds the non-Kaiser point and on the amount of time spent above the non-Kaiser point. In other words, even a load which is just bearly above the non-Kaiser point may cause significant fiber breakage if it is applied for a long time.
As the residual strength is decreased by a damage load, the non-Kaiser point occurs at a lower load level. The effect is thus cumulative. A single severe damage load allows subsequent lesser loads to also produce damage.
In view of the above background, it is apparent that periodic testing may not be sufficient to insure personnel safety. Personnel safety can be better assured by having a continuous monitor employed on-board the aerial truck to warn of loading above the non-Kaiser point before severe damage to the residual strength occurs. It is an object of the present invention to provide such an on-board monitor.
An on-board monitor should be able to identify a damage load by detecting the existence of acoustic emission resulting from fiber breakage for more than a transient period. Since the monitor will be carried on board the truck, it must be able to discriminate acoustic emissions from mechanical and road noise. It must also be able to discriminate the acoustic emissions corresponding to fiber breakage from the acoustic emissions corresponding to the less significant debonding and matrix crazing events. The device should provide an immediate alarm to alert the operating crew when it has detected that a damaged load is occuring. These and further objects of the invention will become apparent as the device is described in more detail.